1. Field of the Invention
The present invention relates to a cooling system for use with a heat source. More specifically, the invention relates to a cooling system for the propulsion system of a high-speed vehicle including a cooling fluid circulating within a closed loop cycle.
2. State of the Art
Aircraft flight speeds in the high supersonic and hypersonic regimes cause severe aerodynamic heating and place severe demands on the structural and thermal capabilities of the engines and airframe. Thus, the performance and mission applications of ramjet and scramjet powered vehicles are dependent on protecting the engines and airframe from these high heat loads encountered at high supersonic and hypersonic speeds. At flight speeds near Mach 4, the air taken on board these vehicles becomes too hot to cool the engines and airframe. Therefore, the vehicle's fuel is conventionally used as the primary coolant.
Cooling systems, which use the latent and sensible heat capacities of aircraft turbine fuels, have long been used on high-performance aircraft. The heat required to heat fuel to its boiling point is known as sensible heat. The heat required to vaporize the fuel is known as the latent heat. Such cooling systems, though, are generally limited to moderate temperature applications to prevent fouling caused by thermal decomposition of the fuel. For example, hydrocarbon fuels may be used for direct cooling of the combustor of an engine. Since fuel overheating of a hydrocarbon fuel may cause coking, combustor wall temperature is restricted to a moderate value in the vicinity of 1000° K. For example, according to U.S. Pat. No. 5,151,171 to Spadaccini et al., incipient coking of JP-7 fuel starts at about 1250° F. (˜950° K). Excess fuel may be used for cooling, providing a greater heat sink and preventing fuel overheating. This use of excess fuel naturally leads to performance deterioration. This supply of additional fuel, beyond the amount needed for combustion, is known as overfueling. Overfueling requires the added weight of the extra fuel, which will not be utilized for propulsion. The extra fuel enters the combustor, but is not fully burned in the combustion process, and is expelled in the form of exhaust.
A “physical heat sink” system is only efficient in cooling a vehicle to flight speeds of about Mach 5.5 to 6. These systems may not be appropriate for use on higher speed vehicles in which relatively higher temperatures will be encountered. Another alternative is to use an endothermic fuel cooling system to provide engine and airframe cooling. Endothermic fuel systems use fuels that have the capacity to absorb an endothermic heat of reaction in addition to sensible and latent heat. As a result, the fuel is capable of absorbing two to four times as much heat as fuels that only absorb sensible and latent heat.
Cracking the fuel is a process of breaking its long-chain hydrocarbon molecules into lighter molecules that absorb heat (an endothermic process). In an endothermic fuel, the heat sink capability of the fuel is made up of its sensible heat plus any net endothermic capacity derived from high fuel dissociation reactions. Hydrocarbon decomposition processes, such as fuel cracking, may be accompanied by carbon formation, or coking. Coking tends to foul heat transfer surfaces, which is undesirable. Thus, there are two parts to calculating the upper limit of a hydrocarbon fuel's heat sink capability: the maximum temperature achievable without the system coking up, and the endothermic capacity of the cracking reactions that can occur.
The “chemical heat sink” of the fuel's endothermic reaction may enable cooling to Mach 6 to 6.5; however, at this point fuel cooling capacity reaches its maximum. Once the maximum combustor wall temperature has been reached, further acceleration is possible only if overfueling techniques are used. The combustor wall/fuel interface temperature is limited to preclude coking. Therefore, such a system might not be adequate to provide sufficient cooling for very high-speed vehicles and can present problems with short catalyst life, catalyst poisoning, special fuel handling and storage considerations, and reaction products having poor combustion properties.
Accordingly, what is needed in the art is a system for cooling high-speed vehicles without overfueling, improving scramjet engine performance and operability and expanding the Mach number capability to, and beyond, Mach 3 to 8+ for hydrocarbon fuels.